Don't know if this is helpful, but I have a Zeiss "Donata" 9x12 plate camera, circa 1930.
I bought it on a trip to Germany, about 25 years ago. It came in the original box, with three plate holders (thin flange).
Just this fall, I found out that roll-film backs were made for these, and I found a RADA (Plaubel) 120 back that fit the camera just fine. It shoots 6x9, but apparently, there was a 6x6 mask also available.
I've shot about 6 rolls of film using this combo, and really like the results.
My Donata has a 13,5 cm lens, so shooting 6x9 gives a bit of a telephoto effect. I use the wire-frame finder, with a series of rubber bands stretched across the frame, to create the 6x9 framelines...
I would really like to find some 9x12 film holders or film-sheaths for my plate-holders, but it's been a blast to finally use this camera after all these years !
Be judicious when searching for roll-film backs: they were also made in now-obsolete film sizes such as 116, 616, etc.
Good luck !
Luddite Frank